This invention relates to a heat treating furnace of the type in which a gas is circulated within a chamber. The gas usually is admitted into the chamber through a valved inlet, is exhausted from the chamber through a valved outlet, and is circulated within the chamber by a fan which is driven for example, by an electric motor. A heat treating furnace of this general type is disclosed in Ispen U.S. Pat. No. 3,219,331.
The density of the gas in the chamber is not constant at all times but instead changes in accordance with factors such as temperature and pressure. Because the density of the gas varies, it is difficult to precisely match the design of the fan and the capacity of the fan motor to the load presented by the gas. In prior furnaces, the fan motor either possesses too much capacity and thus is expensive both in first cost and in operating cost or the motor repeatedly approaches an overload condition and experiences a short service life.